1. Field of the Invention
This invention relates generally to free-machining steel articles and in particular to small diameter wire and bar made from substantially lead-free, free-machining, powder metallurgy steel and to a process for making same.
2. Description of Related Art
Very small, precision parts, such as watch components, are produced by machining small diameter (≦15mm) coils and bars that are made from larger diameter, cold drawn coils and straightened bars. The wire is formed from a cast and wrought steel alloy. To attain good machinability, the coils and barstock are produced from an alloy that contains one or more free-machining additives such as lead (Pb) and/or sulfur (S). The Pb and/or S addition(s) result(s) in the formation of manganese sulfide (MnS) and Pb inclusions, respectively, during the solidification of the cast alloy. The presence of these inclusions decreases the hot and cold workability of the product and also promotes surface defects on the machined part.
Lead is an element that is beneficial to good machinability in the steels used to make precision parts. However, there are disadvantages associated with the production and use of Pb-alloyed steels to make high precision parts. It has proven difficult to control the fineness and even distribution (dispersion) of the Pb in the alloy matrix, the segregation and coalescence of lead inclusions during solidification, and the reproducibility of the process from ingot to ingot and heat to heat. In alloys that contain both Pb and S additions, the Pb and MnS do not interact chemically with each other, although Pb is sometimes becomes physically attached to the MnS inclusions, partially coating them. Because Pb does not dissolve into or bind with iron in steel, it is present only as essentially pure stand-alone inclusions. The use of lead also presents significant health and safety risks that must be addressed during the mill processing operations as well as during the high speed dry machining operations. The high vapor pressure of Pb (i.e. up to about 700 mbar at the temperatures which might be reached during machining) causes Pb to become a major environmental and health problem in dry machining of alloys that contain that element as a free-machining additive.
In steels containing sulfur additions, the sulfur is present substantially in the form of MnS inclusions. Because MnS melts below the solidification temperature range of these steels, it is present as intergranular (interdendritic) inclusions only. Their sizes and distribution depend exclusively on the solidification rate of the ingots. Consequently, it is not possible to avoid the segregation and coalescence of MnS in the as-cast structure and the formation of a wide size spectrum of MnS inclusions, ranging from fine sized inclusions up to very large ones (e.g., ≈125 μm in length) after hot deformation of the alloy. Such a morphology leads to the presence of inclusion-stringers in the free-machining steels. The tolerances (<IT 4-5 Tolerance grade, i.e., <1−2 μm) and surface roughness (Ra of <0.05 μm) required for precision machining of coils and bars made from such steels are very tight. MnS inclusions in resulfurized, lead-bearing 1% C-steels form surface discontinuities at sites of emergence on the surface of such steels, thereby resulting in surface defects that are undesirable in the metal in the machined parts.
The morphologies of the MnS and Pb inclusions of the known steels, also limit the economically achievable precision of the cold drawn wires and straightened bars to a finished tolerance grade ≧IT 6, where IT is the Tolerance grade according to the DIN ISO 286 Standard. Further, the precision, including ovality or out-of-roundness, of the cold drawn wires and cold-drawn and straightened bars, relates directly to the dynamic stability and stiffness or rigidity of the machining process. Therefore, the dynamic stability of the machining process controls the achievable precision and surface quality of the machined parts.